Disc and cone refiners used for manufacturing mechanical pulp are formed of two refiner discs opposite to each other which turn relative to each other and one or both of which is/are rotating. In disc refiners the refiner disc is disc-like and in cone refiners it is conical. The refining surfaces of refiner discs are typically formed of grooves and of protrusions between them, i.e. blade ridges, which will be hereafter called ridges. The shape of these grooves and ridges per se may vary in different ways. Thus, for example, in the radial direction of the refiner disc the refining surface may be divided into two or more circular parts, each of which may comprise grooves and ridges of different shapes. In the same way, the number and density of ridges and grooves as well as their shape and direction in each circle may deviate from each other. Thus, the ridges may be either continuous over the whole length of the refining surface radius or there may be a plurality of successive ridges in the radial direction. A plurality of refiner segments consisting of structures formed of ridges and grooves between them are arranged upon the discs. One of the refiner discs comprises an opening through which the material to be refined is fed into the refiner. The refiner discs are positioned in such a way that the refiner segments form a refiner gap, through which the fibre material is intended to be discharged from the inside, where the ridges of the refiner elements carry out the disintegration. The distance between the refiner discs is longest in the middle of the discs, being reduced towards the outer periphery in order to refine the material gradually.
U.S. Pat. No. 6,311,907 discloses a refiner disc on the refining surface of which some of the ridges in the radial direction of the refiner disc are formed of ridge parts connected to each other in the radial direction of the refiner disc in such a way that between the ridge parts of the refiner disc at their connection point, there is a connecting part that is directed obliquely relative to the direction of the refiner disc radius, which part connects the ridge parts forming the ridge to each other in such a way that the ridge travels windingly from the direction of the inner periphery of the refiner disc to the direction of its outer periphery. The intention of a winding ridge structure is to make the refining more efficient by preventing the material to be refined from moving too rapidly out of the space between the refiner discs towards the outer periphery of the disc. In one embodiment of the publication, the connecting part connecting the ridge parts together is designed to form an adjacent ramp inclined in the direction of the connecting part between the ridge parts, the purpose of the ramp being to facilitate the movement of the material to be refined out of the grooves between the ridge parts of the refining surface to the space between the refiner discs.
It has also been noted that when fibre material is disintegrated to achieve a better final product, it is advantageous to position flow restrictors, i.e. what are called dams, across the grooves of the refiner segments so as to prevent untreated material from getting through the refiner gap. The fibre pulp is forced up from the grooves by the dams and is guided to the treatment between the blade ridges of the refiner segments upon the opposite refiner discs. The more dams there are in the refiner segment, the higher the quality of the fibre pulp obtained from the refining. In practice, however, the number of dams must be kept restricted, because the more dams there are in the refiner segment, the more difficult it is for the water in the refiner gap and the vapour generated due to the high power directed at the disc refiner during the refining to discharge from the refiner gap, and thus the production capacity of the refiner is reduced. In addition, the vapour pressure generates great axial forces between the refiner segments, particularly in the outer part of their periphery, which loads the refiner bearings and thus also restricts the runnability of the refiner. High vapour pressure also causes bending of refiner segments so that the segments loose their parallelism.